Cytochrome reductase is the middle section of the mitochondrial respiratory chain, passing electrons between the dehydrogenases and cytochrome oxidase and conserving energy in the form of an electrochemical proton gradient across the mitochondrial membrane. We have collected data to 4.5 E (complete to about 5 E) on heavy atom derivatives of the hexagonal crystal produced by co-crystallizing with a tightly-binding synthetic inhibitor containing a mercury atom. We have located a heavy atom site in the TAMM derivative of beef crystals. We have developed a freezing protocol for the beef crystals and crystallized the enzyme from 3 new sources, two of which are very promising. We need SR for more screening and data collection.